X86: Remove hack in shouldRewriteCopySrc for subregister handling

llvm/lib/CodeGen/TargetRegisterInfo.cpp

@@ -420,7 +420,10 @@ static bool shareSameRegisterFile(const TargetRegisterInfo &TRI,
                                   const TargetRegisterClass *SrcRC,
                                   unsigned SrcSubReg) {
   // Same register class.
-  if (DefRC == SrcRC)
+  //
+  // When processing uncoalescable copies / bitcasts, it is possible we reach
+  // here with the same register class, but mismatched subregister indices.
+  if (DefRC == SrcRC && DefSubReg == SrcSubReg)
     return true;
 
   // Both operands are sub registers. Check if they share a register class.

llvm/lib/Target/X86/X86RegisterInfo.cpp

@@ -224,21 +224,6 @@ X86RegisterInfo::getPointerRegClass(const MachineFunction &MF,
   }
 }
 
-bool X86RegisterInfo::shouldRewriteCopySrc(const TargetRegisterClass *DefRC,
-                                           unsigned DefSubReg,
-                                           const TargetRegisterClass *SrcRC,
-                                           unsigned SrcSubReg) const {
-  // Prevent rewriting a copy where the destination size is larger than the
-  // input size. See PR41619.
-  // FIXME: Should this be factored into the base implementation somehow.
-  if (DefRC->hasSuperClassEq(&X86::GR64RegClass) && DefSubReg == 0 &&
-      SrcRC->hasSuperClassEq(&X86::GR64RegClass) && SrcSubReg == X86::sub_32bit)
-    return false;
-
-  return TargetRegisterInfo::shouldRewriteCopySrc(DefRC, DefSubReg,
-                                                  SrcRC, SrcSubReg);
-}
-
 const TargetRegisterClass *
 X86RegisterInfo::getGPRsForTailCall(const MachineFunction &MF) const {
   const Function &F = MF.getFunction();

llvm/lib/Target/X86/X86RegisterInfo.h

@@ -70,11 +70,6 @@ class X86RegisterInfo final : public X86GenRegisterInfo {
   getLargestLegalSuperClass(const TargetRegisterClass *RC,
                             const MachineFunction &MF) const override;
 
-  bool shouldRewriteCopySrc(const TargetRegisterClass *DefRC,
-                            unsigned DefSubReg,
-                            const TargetRegisterClass *SrcRC,
-                            unsigned SrcSubReg) const override;
-
   /// getPointerRegClass - Returns a TargetRegisterClass used for pointer
   /// values.
   const TargetRegisterClass *

llvm/test/CodeGen/X86/pr41619_reduced.mir

@@ -0,0 +1,27 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 5
+# RUN: llc -mtriple=x86_64-- -mattr=+avx2 -run-pass=peephole-opt -o - %s | FileCheck %s
+
+# When trying to coalesce the operand of VMOVSDto64rr, a query would
+# be made with the same register class but the source has a
+# subregister and the result does not.
+---
+name:            uncoalescable_copy_queries_same_regclass_with_only_one_subreg
+tracksRegLiveness: true
+isSSA:           true
+body:             |
+  bb.0:
+    liveins: $rax
+
+    ; CHECK-LABEL: name: uncoalescable_copy_queries_same_regclass_with_only_one_subreg
+    ; CHECK: liveins: $rax
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: [[COPY:%[0-9]+]]:gr64 = COPY $rax
+    ; CHECK-NEXT: [[COPY1:%[0-9]+]]:vr128 = COPY [[COPY]].sub_32bit
+    ; CHECK-NEXT: [[VMOVSDto64rr:%[0-9]+]]:gr64 = VMOVSDto64rr [[COPY1]]
+    ; CHECK-NEXT: RET 0, implicit [[VMOVSDto64rr]]
+    %0:gr64 = COPY $rax
+    %1:vr128 = COPY %0.sub_32bit
+    %2:gr64 = VMOVSDto64rr %1
+    RET 0, implicit %2
+
+...